A common configuration for semiconductor processing equipment utilizes a number of different processing chambers accessible from a central wafer storage and handling ("transfer") chamber. FIG. 1 schematically illustrates a semiconductor processing system having such a configuration. Wafers are typically loaded into the processing system in wafer cassettes and then the wafers are individually transferred from the cassettes to a wafer storage elevator within the central transfer chamber. A wafer transport robot moves individual wafers from the wafer storage elevator through valves in the transfer chamber wall, into the different processing chambers, and then between chambers to effect different processing steps. Movement of semiconductor wafers within processing equipment is accomplished using automated wafer handling techniques to allow for the complete automation of the fabrication process. In addition to the system illustrated in FIG. 1, many other configurations of wafer processing equipment use automated wafer handling techniques.
Because wafer processing occurs in a vacuum or near vacuum environment and because cost considerations necessitate fast pumping times to facilitate high wafer throughput, the total volume of a wafer processing system is limited. Consequently, the clearances and tolerances within wafer processing equipment are typically limited by space considerations. For example, one type of wafer storage elevator stores wafers in a recess that allows only a few millimeter clearance to either side of the wafer. Misaligned wafers may be dislodged from the wafer storage elevator or may be damaged in other ways. Considering the complexity of semiconductor devices and the number of devices on each wafer, each wafer start represents a substantial investment and any level of wafer breakage is undesirable. Accordingly, wafer transfers must be precise. Normally, stepper motor driven wafer transport robots under computer control are capable of repeatedly transporting wafers through a processing system with great precision. However, the effectiveness of such wafer handling techniques can be greatly diminished if the initial wafer position with respect to the wafer transport robot is not known accurately.
Some types of wafer processing equipment use one or more techniques to measure a wafer's position and to ensure that the wafer is situated at a predefined position. For example, U.S. Pat. No. 4,819,167 to Cheng, et al., entitled "System and Method for Detecting the Center of an Integrated Circuit Wafer," describes a wafer processing system which includes a sensor array for determining the position of a wafer as it is loaded from an external cassette onto an internal wafer storage elevator. The Cheng wafer positioning system ensures that wafers loaded into the processing equipment are accurately positioned on a given level of the wafer storage elevator. Using a system in accordance with the teachings of the Cheng patent ensures that wafers stored within the wafer storage elevator are initially at a well defined position with respect to the wafer transport robot when the wafers are accessed by the wafer transport robot.
Systems which incorporate the teachings of the Cheng patent nevertheless experience unacceptable levels of wafer breakage. This is likely caused by inaccurate wafer accessing operations or by wafers being dislodged from their nominal position either during processing or transport. Thus, further measures are desirable to ensure that wafers are at their predetermined positions. Some wafer processing systems use capacitive position sensors within the blade of the wafer transport robot to detect if the wafer is improperly seated on the blade. Such capacitive systems are often inaccurate and provide too unreliable of a signal to detect most wafer misalignment problems.
A second type of centerfinding system is described in U.S. patent application Ser. No. 07/975,197, entitled "System and Method for Automated Positioning of a Substrate in a Processing Chamber," which lists Shmookler, et al., as inventors (hereinafter, the "Shmookler system"). application Ser. No. 07/975,197 was filed on Nov. 12, 1992, now abandoned, and is assigned to the assignee of the present invention. The Shmookler system is a wafer centerfinding system which uses four photoelectric position sensors to locate the center of a wafer as the wafer is moved between processing chambers. An array of optical sources are disposed above the central wafer transport chamber and a corresponding array of optical detectors are disposed below the chamber. The illustrated sensor array allows the identification of wafer positions, but requires that the wafer transport chamber be optically accessible from both the top and the bottom of the chamber. This photoelectric sensor array is arranged so that the light travels along a path generally perpendicular to the plane in which the wafer is transported. In practice, this type of sensor geometry may lead to erroneous position information due to multiple reflections from the surfaces of the top and bottom chamber covers and from the wafer. To compensate for such erroneous position data, the Shmookler system samples more data points than necessary, discarding data that does not fall within expected limits.
The Shmookler system uses a data collection scheme which relies on particular points on the edge of a wafer crossing the four sensor array in a particular order. This scheme works well when wafers are positioned near to their nominal position. However, wafers that are in danger of breaking in the course of a transport operation may be dislodged from their nominal position by a large amount. For such substantially misaligned wafers, the Shmookler system will not appropriately identify the wafer position, and wafers that are substantially out of position may consequently be broken.